Method for imaging the relative motion of skeletal segments

ABSTRACT

Apparatus provided for the measurement of skeletal joint motion in a subject which comprises a passive motion device, an imaging device and a processing system incorporating a means for real time digital sampling of images of moving joints, means for recognising templates attributed to individual bones and tracking these automatically using cross-correlation functions and means for geometric transformation of the positional data to graphically display their relative motion over time. Also provided is a method for the automated measurement of the relative motion of skeletal structures in vivo using such apparatus and a method for the diagnosis of a pseudoarthrosis in a subject which comprises the use of such apparatus.

The present invention relates to an automated system for monitoring themovement of bones in the skeleton of a subject, with particularreference to the bones in patients after surgery.

The skeleton is the support system of land animals and its joints arekey to its structural integrity in everyday life. Examining thisintegrity for the purpose of understanding malfunction in the livingorganism, without penetrating the surface, has hitherto been aninsurmountable problem, preventing accurate diagnosis and informedtreatment. This has meant that the functional integrity of joints,especially the spinal joints, could not be assessed in living subjectswithout resorting to invasive procedures. Spinal fusions, often a lastresort for intractable back pain, could not be inspected for theirsuccess without revision surgery, and suspected disruption of ligamentscould not be objectively assessed.

Attempts to overcome this difficulty by placing measuring devices on thesurface of body segments, and recording their displacements duringmovement of the body were unsatisfactory because it was surface (skin)rather than bone motion that was recorded—especially in relation to thesegments of the spine. The use of plain X-rays was also unsatisfactorybecause only the beginning and end of the motion could be recordedwithout giving a prohibitive radiation dose. Attempts involvingcineradiography and videofluoroscopy allowed the whole range of motionto be seen on film or videotape, but not measured. Furthermore, markinga sufficient number of the images in a motion sequence manually, inpursuit of such measurement, was too laborious to support a method foruse in clinical settings. See, for example, U.S. Pat. No. 5,090,042.Additionally, voluntary motion of joints adds the confounding factor ofthe stabilising influence of the muscles, concealing any abnormality ofthe joint ligaments or other passive elements, notably theintervertebral discs.

There is, therefore, a need for a system that provides a means forproducing real-time image generation of the motion of the bones in asubject than can objectively measure the functional integrity of jointtissues with the minimum of invasiveness.

The present invention makes it immediately possible to use x-rayintensifier technology, or its successors (e.g. real-time magneticresonance or other imaging), to carry out these procedures byobjectively measuring the functional integrity of joint tissues with aminimum of invasiveness. Its immediate application is to the detectionof failed spinal fusions, avoiding the necessity for a second operationto inspect the integrity of the original graft.

According to a first aspect of the invention, there is providedapparatus for the measurement of skeletal joint motion in a subjectcomprising:

-   -   (a) a passive motion device which comprises a horizontal        platform base and a horizontal passive motion platform composed        of a horizontal static platform which is rigidly connected to        the upper lateral surface of the platform base and a horizontal        laterally movable platform which is flexibly connected to the        static platform or to the upper surface of the platform base, in        which the static platform is adjacent to the laterally movable        platform which together both form the passive motion platform,        in which the movement of the laterally movable platform is        driven by a motor attached to the platform base where movement        of the laterally movable platform is achieved by means of a        control arm that operably connects the laterally movable        moveable platform to the motor;    -   (b) an imaging device;    -   (c) a processing system which comprises a computer incorporating        a means for real time digital sampling of images of the moving        joints, means for recording time code and data from the passive        motion platform; means for storage of these images at high        resolution; means for recognising templates attributed to        individual bones and tracking these automatically using        cross-correlation functions; and means for geometric        transformation of the positional data to graphically display        their relative motion over time.

The apparatus for the measurement of skeletal joint motion allows forthe accurate measurement of movement in skeletal structures through theoperation of the passive motion device which permits the joints to bemoved at a controlled rate within patient tolerances and through a rangeappropriate for measurement using the means contained within theprocessing system.

The horizontal platform base may be a table or similar construction topermit stable operation of the device. Suitable tables include tablesused for X-ray purposes or imaging purposes in a medical environment.

The passive motion platform is composed of a horizontal static platformand a horizontal laterally movable platform. The static platform can besecurely fixed to the platform base through its lower lateral surface.The static platform co-operates with the laterally movable platform soas to provide a horizontal surface on which a patient for observationmay be positioned.

The laterally movable platform may be flexibly connected to the staticplatform or to the upper surface of the platform base so as to permitmovement of the movable platform in a horizontal plane. The moveableplatform or swing platform may be moved through the action of a motorattached to the platform base. The movement of the movable platform maydescribe an arc sufficient to cause movement of the body of the patientto be observed. In some embodiments of the invention, the laterallymovable platform may be superposed or placed on a support which lies onthe upper surface of the platform base. Such a support may assist instabilising the motion of the movable platform in use. Since therotation of the laterally movable platform may be rotation around afixed point, the support may take the form of an arc (or circularsegment), may be a protractor.

The movement of the laterally movable platform is controlled by a motorattached to the platform base that acts through a control arm. The motorcan be hand-operated or powered by electricity. The control arm may becomposed of a drive cylinder and a drive rod. Preferably, the drivecylinder has a means for setting the range of movement of the movableplatform. The motor may be suitably controlled from behind the x-rayconsole.

The imaging device is preferably an X-ray tube and image intensifierwith dosage control or a magnetic resonance scanner capable of real-timeimaging of the joint being examined or any other imaging device capableof providing adequately resolved images.

The processing system as defined above comprises a computerincorporating means for recording and analysing data. Such means forreal time digital sampling of images of the moving joints may be imageprocessing software capable of manipulating sequential images, forexample “ImagePro”. As an alternative to sampling analogue output formimages, the direct sampling of digital format images may also bepreferred. Data obtained from the intensifier in a digital format can beaccessed by DICOM. The means for recording time code and data from thepassive motion platform may be a framegrabber card compatible with thecomputer image processing software and a time code generator connectedto the computer peripherally (for example a FOR.A TGR2000). The imagesgenerated may be stored at high resolution on the hard drive of thecomputer or on a suitable data carrier, for example a compact disc.

The means for recognising templates attributed to individual bones andtracking these automatically using cross-correlation functions may besoftware for complex mathematical transformations, for example “Matlab”.

The means for geometric transformation of the positional data tographically display their relative motion over time may be a statisticalspreadsheet software program such as Microsoft “Excel”. This may includeaveraging repeated trackings to optimise reliability.

The processing system may additionally comprise a means forautomatically correcting image dimensions for any distortion contributedby the image intensifier.

In general the computer hardware and image analysis software will havesufficient on-line memory, bit depth and processor speed to sample,affix time code and hold multiple high quality images; sufficientdigital storage to retain and replay image sequences; triggering andcontrol linkages to the passive motion and imaging devices; imagecalibration to correct systematic image distortion; tracking code andalgorithms suitable for registering the relative positions of a templateplaced around a number of adjacent bone images in series throughoutmotion sequences; linkage of these outputs to graphical and statisticalprograms.

The passive motion platform for lower spinal examinations consists oftwo flat linked surfaces, the static platform and the laterally movableplatform, made of radiolucent material on which the subject lies. Thisis driven by a motorised arm linked to the computer acquisition systemduring imaging. The tracking of individual bone images accessed directlyfrom an intensifier or other digital imaging output uses much of all theimage data for each bone using algorithms based on picture elementcorrelations that can be improved upon by the amount of data availableas pixel depth and density. This level of control over image quality isnot possible by videofluoroscopic methods, because of the degradation ofimages caused by using videotape as a storage intermediary. The problemsof image degradation by any metallic implants or other artefacts isresolved by using templates to define suitable areas of bone image fortracking and the automated data outputs can be averaged to achievegreater reliability. These are fundamental advances on current surfaceor imaging methods that do not acquire sufficient data, with sufficientspeed or with a sufficient degree of automation and to not measure theresponses of the passive holding elements to motion.

It is a central object of the invention to provide an automated systemthrough which the relative motion of images of skeletal structures canbe measured in vivo. These range from spinal to limb girdle joints andare suited particularly to the discrimination of movement in joints thathave been the subject of surgical fusion. A wider application is,however, the ability of the invention to reveal the motioncharacteristics of non-fused joints. It is intended that the inventionallow moving images of bones to be acquired within a viewing field whichaddresses an area of interest selected for relative motion assessmentwith minimal invasiveness over a period of under one minute (if X-raysare used). In the case of X-ray generated images, this incorporatesmethods for gonadal shielding, filtration and intensifier flarereduction as well as patient stabilisation, procedure rehearsal andability to stop the procedure if desired. Real-time digital acquisitionwith superimposed time code and storage of the images for playback andsubsequent automated motion analysis is implicit in the invention. Theinvention outputs numerical or graphical data depicting the relativemotion of adjacent bones. These data can be statistically analysed forrepeatability and automatically re-calculated and averaged as a means oferror reduction. They can also be transformed to display differentindices of the motion (e.g. angular change, translational change, oraxis of rotation). It is intended that the outputs take the formgraphical displays of the motion features of interest for the attentionof clinicians.

The apparatus of the invention can be used to measure movement in theskeletal joints of a subject. The subject may be any animal having aninternal bony skeleton, preferably an animal with is a mammal. Theinvention may find greatest utility in the fields of human andveterinary medicine. In veterinary medicine, the method may find use inthe treatment of domestic pets as well as to agricultural or zoologicalanimals.

The skeletal joints that can be measured include, but are not limited tothe intervertebral linkages of the cervical (neck), thoracic (upperback) and lumbar (lower back) spines In humans, the cervical vertebraeare also known as vertebrae C1 to C7, the thoracic vertebrae as T1 toT12, and the lumbar vertebrae as L1 to L5.

So for example relative motion of lumbar vertebrae L1 to L2, L2 to L3,L3 to L4 or L4 to L5 can be measured simultaneously or separately.

The devices of the present invention differ therefore in certain keyrespects from those of the prior art. Principally, there is thedigitisation of sampled images which provides the enhanced reliabilityof the methods carried out using the apparatus. The improved reliabilityis the result of the ability to handle and process large amounts ofinformation which is not seen in the prior art.

According to a second aspect of the invention, there is provided amethod for the automated measurement of the relative motion of skeletalstructures in vivo, comprising the steps of:

-   -   (i) positioning the subject on a passive motion device as        defined in accordance with the first aspect of the invention.;    -   (ii) initiating the imaging procedure of the subject positioned        on the passive motion device and collecting image data using an        imaging device;    -   (iii) sampling the data collected by the imaging device into the        processing system and superimposing time code on the images;    -   (iv) tracking templates marked on individual bone segments at        the start of the motion sequence;    -   (v) transforming the results of tracking to reflect the changing        spatial relationship between image segments; and    -   (vi) presenting the output in graphical form.

This aspect of the invention therefore provides methods for acquiringimages and analysing the motion of adjacent skeletal structures.

In a preferred embodiment of this aspect of the invention, a calibrationstep is carried out prior to the method described above. Prior toimaging the skeletal structure, calibration of the computer is achievedby imaging objects of known dimensions in order to allow for anygeometric distortion inherent in the imaging device.

The methods may also include a further optional step of measuring theforces involved in the motion of the skeletal structures by measuringthe mechanical resistance to the table motion.

Simple adaptation make the method suitable for use with other joints,including those of the cervical spine (neck). The subject is alsopreferably shielded from x-ray radiation by means of lead shieldingmaterial to minimise the dose received.

Methods in accordance with this aspect of the invention can be appliedto any joint, safely, reliably and comprehensively, using any imagingsystem capable of real-time image generation. Such methods can beoperated by a radiographer without specific medical training. Themethods involve stabilising two adjacent body segments in a mechanicaldevice that moves a joint passively, while briefly imaging this motionin real time. The motion of adjacent bones is tracked by applyingdigital image processing algorithms to their image sequences andoutputting the relative motion data graphically for the inspection ofclinicians. Any systematic distortion within the device can becalibrated for and subjected to corrective transformation as a part ofprocessing. Transformation may include dimensions of structures, theirrotation, translation and centres of rotation.

Advantages of these methods over the prior art can be summarised asfollows. The invention enables the ability to examine side-bendingmotion, not just flexion-extension in a patient. It is possible tomeasure many hundreds of data points (typically 100 to 500) rather thanthe more limited number measurable according to prior art methods thattypically can only measure 6 data points. As a result, methods of thepresent invention enable the ability to analyse segmental motion whichoccurs at small parts of the range as, for example, in a failed fusion(for example see, FIG. 3 which shows a graph of a patient test with 120data points). Such data collection requires direct digitisation of thesample images. The ordinary use of videotape is not sufficient as itdegrades images so seriously that they cannot be tracked and attempts todevise an imaging technique using videotape remain manual anduneconomic. The use of the passive motion device of the presentinvention controls the motion of the patient's body and thereforediagnosis depends far more on the device than the coordination abilitiesof the patient. It is also possible to measure the forces involved inthe joint movement by measuring the mechanical resistance to the tablemotion.

Methods of the present invention therefore enable the objective andaccurate measurement of the small movements between vertebrae. Inparticular, the methods facilitate the investigation of suspected failedspinal stabilisation surgery (pseudoarthrosis) and/or suspected damageto intervertebral soft tissue linkages

Such methods permit accurate measurement of the small ranges ofsegmental motion throughout the range of flexion/extension andside-bending of the spine using fluoroscopy to image the spine inpassive motion. The sequential images thus obtained are then stored ondisc. Computer software as described herein is used to track themovements of the vertebrae.

The methods can be considered to comprise the following broad elements:

-   -   (i) data acquisition—fluoroscopy of the lumbar spine region in        flexion/extension and side-bending and digitisation of the        images;    -   (ii) data analysis—registering the positions of the vertebrae        and tracking the vertebrae throughout the sequence of images;    -   (iii) generation of a report with objective evidence (for or        against) of the existence of fusion or of pseudoarthrosis in a        patient.

The process of data acquisition is undertaken using a device of thepresent invention as described herein. Patients lie supine on an x-raytable of a passive motion device as defined above for side-bendingsequences of movement. Intervertebral motion is therefore passive. Thisreduces error in analysis due to muscle guarding or unilateral weakness.Patients then lie on their side and the procedure is repeated forflexion/extension. The lower half of the x-ray table (the swingplatform) can swing to a maximum of +/−40 degrees of motion.

After labelling the image file with the patient's details theradiographer views the series for quality. The radiographer then usesthe first image in the series to mark templates around the bony segmentsof interest. The first frame of the sequence is selected and theappropriate vertebrae are identified (i.e. for L4/L5 intervertebralangles, L4 and L5 would be marked). This involves marking four referencepoints (typically the corners) around each vertebral body to make atemplate. This is done for each vertebral body so as to include as muchof the bone as possible with minimal surrounding soft tissue.

In instances, where there are metallic implants present in the spine(for example as shown in FIG. 2(b)), there may be more than 4 pointsused to create a template.

The saving of images acquired in the data acquisition process is asfollows. The output from the image intensifier may be linked to softwarein the computer. The images can then be captured in real time (forexample 25 frames per second). The maximum possible digital informationcan then be obtained to increase sensitivity of the vertebral tracking(10 bit depth, 1024 by 1024 pixel density). Each image can be stored asa “.tiff” file and typically there can be 500 images per sequence (or1000 images per patient).

According to a third aspect of the invention there is provided a methodfor the diagnosis of a pseudoarthrosis in a subject, the methodcomprising analysing the relative motion of skeletal structures in thepatient according to a method of any one of claims 5 to 7.

Preferred features for the third and subsequent aspect of the inventionare as for the first aspect mutatis mutandis.

An apparatus for the measurement of skeletal joint motion in a subjectin accordance with the present invention is described in FIGS. 1(a) and1(b). An apparatus is shown which comprises a passive motion device (1)having a horizontal platform base (23) and a horizontal passive motionplatform (25). The horizontal passive motion platform (25) is situatedon the horizontal platform base (23). The horizontal passive motionplatform (25) is composed of a horizontal static platform (7) which isrigidly connected to the upper lateral surface of the platform base anda horizontal laterally movable platform (5) which is flexibly connectedto the static platform or to the upper surface of the platform base, inwhich the static platform is adjacent to the laterally movable platformwhich together both form the passive motion platform, in which themovement of the laterally movable platform is driven by a motor (9)attached to the platform base where movement of the laterally movableplatform is achieved by means of a control arm (11, 13), composed ofdrive (13) and drive cylinder (11) that operably connects the laterallymovable moveable platform to the motor. An imaging device (22, 21) ispositioned around the device (1) such that movement of the skeletaljoint in the subject can be imaged. The imaging device is suitably anX-ray tube (22) and an image intensifier (21). The device (1) has aprotractor base (3) underneath the laterally movable platform (5) whichis also provided with a runner (27). The device (1) also containslinkages to a patient control or panic button (15), radiographer controlpanel (17) which may be an X-ray console where the imaging device is anX-ray tube, and a computer and time code generator (19).

In use, the device comprises of a static platform and a swing platform.The latter is sited atop a second static platform which also serves as aprotractor to indicate the arc of motion which it describes and overwhose centre is to be sited the joint of interest. The swing platformarticulates with the static platform as a two-dimensionalball-and-socket joint and runs over the static protractor section on lowfriction wheels (runners). Both the static and the swing platforms aremade of radiolucent material and have extensions on which to attach adrive mechanism.

The drive mechanism consists of an electric motor which drives a rod inand out of a cylinder in order to move the swing portion of the tablerelative to the static platform. This is controlled by a computer chipthrough which the rate and range of motion can be pre-set and is linkedto an ammeter by which the resistance to the motion can be measured.

The motor is operated by a radiographer or assistant with a patientoverride switch. The motor is also connected to a time code generatorsuch that initiating movement of the platform trips the registration oftime code on the images acquired.

The central X-ray beam passes from the X-ray tube to the imageintensifier through the joint level of interest during the motion of theswing platform. As the motion progresses the area of interest is kept inthe central X-ray beam by the radiographer, and appropriate gonadalshielding and flare reduction is applied.

The conventional path of motion is from the neutral position to fullrange in one direction, to full range in the opposite direction and backto neutral.

The invention will now be further described by way of reference to thefollowing Examples and Figures which are provided for the purposes ofillustration only and are not to be construed as being limiting on theinvention. Reference is made to a number of Figures in which:

FIG. 1(a) shows the passive motion platform in its top elevation whereit is sited atop an X-ray table.

FIG. 1(b) shows the passive motion in its side elevation as would beviewed from the X-ray console with a patient undergoing the imaging ofvertebral joint motion in the lower spine in the sagittal plane.(Turning the patient to the supine position would allow side-bending, orcoronal plane, motion.)

FIG. 2(a) shows the three linked components of the system, being thepassive motion platform, the X-ray machine or other imaging device andthe computer acquisition and analysis system

FIG. 2 b shows an X-ray image of a vertebra with implanted metal screwsand rods and with the outline of a template which denotes the areas ofbony image enclosed within the template for automatic tracking.

FIG. 3(a) shows a line graph in which in which is shown the results oftracking the angular motion of one intervertebral linkage (2 consecutivevertebrae) through a full side-bending range. The x-axis denotes thenumber of increments of motion between images registered by the trackingsystem. The y-axis denotes the magnitude of the angles between the onepair of vertebrae in side-bending (coronal plane motion) with, byconvention, left side-bending being the positive direction and rightside-bending the negative.

FIG. 3(b) shows an example of automated tracking results for an averageof ten registrations of a series of mobile intervertebral joints througha full side-bending range for 4 vertebrae simultaneously (vertebrae L2to L3, L3 to L4, and L4 to L5)

FIG. 4 shows the normal intervertebral angles of vertebrae L4/L5 duringpassive side-bending motion. The error bars express a 95% confidenceinterval.

FIG. 5 shows successfully fused vertebrae L4/L5 during side bendingmotion

FIG. 6 shows abnormal movement during side-bending in a bone model ofvertebrae that have been surgically stabilised. This is indicative of apseudoarthrosis.

FIG. 7 shows the results from FIGS. 4, 5 and 6 combined whichdemonstrates the ability of the methods and devices of the presentinvention to track vertebrae and to calculate intervertebral anglesleading to accurate clinical diagnoses.

EXAMPLES Example 1 Imaging of a Patient with Intractable Spinal Pain

Data Acquisition

Typically, a patient with chronic intractable spinal pain will bereferred for the investigation. The patient will normally attend theX-ray department as an outpatient and will enter an imaging suite underthe direction of a radiographer and an assistant. All components of thedevice, which is portable, will be in place when the patient enters.

The patient will be familiarised with the action of the passive motionplatform by demonstration and then will be helped to lie on it in theprone or supine position. The swing platform will be moved and thepatient's acceptance of the motion determined. The range of motionachievable will be decided by discussion and tested without imaging toensure it is well tolerated. Devices for gonadal protection and thereduction of any intensifier flare will then be placed on the passivemotion platform.

The radiographer will centre the level of interest and configure theimaging parameters for the exposure. The assistant will prepare theacquisition system to sample digital images in real time and imprinttime code on them.

On a countdown the imaging and digital sampling will begin. The swingsection of the passive motion platform will then describe the full rangeof the motion previously rehearsed with the patient. If the patientwishes to stop the motion, they will press a hand-held control whichwill return the position to neutral. The acquisition time is normallyunder 30 seconds.

After the image sequence has been acquired, the patient may be imaged inanother plane. If so the same procedure will be followed. At the end ofthe imaging session the patient is helped from the passive motionplatform and leaves.

Data Analysis

After labelling the image file with the patient's details theradiographer views the series for quality. The radiographer then usesthe first image in the series to mark templates around the bony segmentsof interest. The first frame of the sequence is selected and theappropriate vertebrae are identified (i.e. for L4/L5 intervertebralangles, L4 and L5 would be marked). This involves marking four referencepoints (typically the corners) around each vertebral body to make atemplate. This is done for each vertebral body so as to include as muchof the bone as possible with minimal surrounding soft tissue.

In instances, where there are metallic implants (for example as shown inFIG. 2(b)), there may be more than 4 points used to create a template.

The tracking of images is done automatically using cross-correlationcodes and the results held on a spreadsheet as angular or translationalmotion data, or a transformation thereof. The tracking process isrepeated with new templates to determine the repeatability ofmeasurement. High quality images with high repeatability will undergofewer tracking sequences than lower quality ones. The data from thelatter may be averaged over several trackings to achieve reliableresults.

The co-ordinates of vertebral movement are then converted into vertebralangles using mathematical software. Intervertebral angles are obtainedby subtracting vertebral angles from two adjacent vertebrae.

Each sequence of vertebral motion is analysed five times. In otherwords, the whole procedure of drawing templates is repeated five times.The mean value for intervertebral angles is calculated and representedgraphically as evidence (for or against) conclusive fusion or conclusivepseudoarthrosis.

The results of a typical session are graphically displayed in FIG. 3(a).

Example 2 An Objective Spinal Imaging Assessment of the Integrity ofLumbar Spine Stabilisation Grafts

The prospect of a second operative procedure following an apparentlyunsuccessful spinal fusion is an unwelcome one. The method of thepresent invention described herein combines sufficiently reducedoperator interaction with acceptable error limitation to beoperationally useful as a tool for reporting findings about graftintegrity for spinal surgeons.

Methods and results: The measurement of lumbar inter-vertebral coronaland saggital plane motion ii: vivo using this technique is in 3 stages:

-   -   Fluoroscopic screening of patients lying on a passive motion        table    -   Co-ordinated real-time digital acquisition of the intensifier        images.    -   Registration of the images of each vertebra by templates which        are automatically tracked and whose output is converted to        inter-vertebral kinematic parameters and averaged for display        and reporting.

Results are currently displayed as inter-vertebral angles throughout themotion (FIG. 4) that indicate whether or not solid fusion has beenachieved, the Instrument Measurement Error is quantifiable and will varywith image quality, but can be improved by averaging. The technology isapplicable to any imaging system of sufficient speed and resolution andmay, for example, be used with MR in the future.

FIG. 5 shows stable fusion in side bending in a fused bone model usingthe device. FIG. 6 shows predicted results from a hypotheticalpseudoarthrosis. FIG. 7 combines the graphical representations ofnormal, fused and pseudoarthrosis from FIGS. 4, 5 and 6 for comparison.

1-8. (canceled)
 9. An apparatus for the measurement of skeletal jointmotion in a subject comprising: a) a passive motion device whichcomprises a horizontal platform base and a horizontal passive motionplatform composed of a horizontal static platform which is rigidlyconnected to the upper lateral surface of the platform base and ahorizontal laterally movable platform which is flexibly connected to thestatic platform or to the upper surface of the platform base, in whichthe static platform is adjacent to the laterally movable platform whichtogether both form the passive motion platform, in which the movement ofthe laterally movable platform is driven by a motor attached to theplatform base where movement of the laterally movable platform isachieved by means of a control arm that operably connects the laterallymoveable platform to the motor; b) an imaging device; and c) aprocessing system which comprises a computer incorporating a means forreal time digital sampling of images of the moving joints, means forrecording time code and data from the passive motion platform; means forstorage of these images at high resolution; means for recognisingtemplates attributed to individual bones and tracking theseautomatically using cross-correlation functions; and means for geometrictransformation of the positional data to graphically display theirrelative motion over time.
 10. An apparatus as claimed in claim 9, inwhich the imaging device is an X-ray tube and image intensifier withdosage control.
 11. An apparatus as claimed in claim 9, in which theimaging device is a magnetic resonance scanner.
 12. An apparatus asclaimed in claim 9, in which the laterally movable platform is situatedon a support which lies on the upper surface of the platform base. 13.An apparatus as claimed in claim 12, in which the imaging device is anX-ray tube and image intensifier with dosage control.
 14. An apparatusas claimed in claim 12, in which the imaging device is a magneticresonance scanner.
 15. A method for the automated measurement of therelative motion of skeletal structures in vivo comprising: i)positioning the subject on a passive motion device as defined in claim9; ii) initiating the imaging procedure of the subject positioned on thepassive motion device and collecting image data using an imaging device;iii) sampling the data collected by the imaging device into theprocessing system and superimposing time code on the images; iv)tracking templates marked on individual bone segments at the start ofthe motion sequence; v) transforming the results of tracking to reflectthe changing spatial relationship between image segments; and vi)presenting the output in graphical form.
 16. A method according to claim15, in which the imaging device is an X-ray tube and image intensifierwith dosage control.
 17. A method according to claim 15, in which theimaging device is a magnetic resonance scanner.
 18. A method accordingto claim 15, in which the laterally movable platform is situated on asupport which lies on the upper surface of the platform base.
 19. Amethod according to claim 18, in which the imaging device is an X-raytube and image intensifier with dosage control.
 20. A method accordingto claim 18, in which the imaging device is a magnetic resonancescanner.
 21. A method according to claim 15, in which a calibration stepis carried out prior to the method steps i) to vi).
 22. A methodaccording to claim 15, in which the relative motion of lumbar vertebraeL3 to L3, L3 to L4 and L4 to L5 are tracked simultaneously orseparately.
 23. A method according to claim 21, in which the relativemotion of lumbar vertebrae L3 to L3, L3 to L4 and L4 to L5 are trackedsimultaneously or separately.
 24. A method for the diagnosis of apseudoarthrosis in a subject, the method comprising analysing therelative motion of skeletal structures in the patient according to themethod of claim
 15. 25. A method according to claim 24, in which theimaging device is an X-ray tube and image intensifier with dosagecontrol.
 26. A method according to claim 24, in which the imaging deviceis a magnetic resonance scanner.
 27. A method according to claim 24, inwhich the laterally movable platform is situated on a support which lieson the upper surface of the platform base.
 28. A method according toclaim 27, in which the imaging device is an X-ray tube and imageintensifier with dosage control.
 29. A method according to claim 27, inwhich the imaging device is a magnetic resonance scanner.
 30. A methodaccording to claim 24, in which a calibration step is carried out priorto the method steps i) to vi).
 31. A method according to claim 24, inwhich the relative motion of lumbar vertebrae L3 to L3, L3 to L4 and L4to L5 are tracked simultaneously or separately.
 32. A method accordingto claim 30, in which the relative motion of lumbar vertebrae L3 to L3,L3 to L4 and L4 to L5 are tracked simultaneously or separately.